What is blood pressure and how is it measured?

The heart supplies the organs and tissues of the body with blood. With every beat, it pumps blood into the large blood vessels of the circulatory system. As the blood moves around the body, it puts pressure on the walls of the vessels. Blood pressure readings are made up of two values:

  • Systolic blood pressure is the pressure when the heart beats – while the heart muscle is contracting (squeezing) and pumping oxygen-rich blood into the blood vessels.
  • Diastolic blood pressure is the pressure on the blood vessels when the heart muscle relaxes. The diastolic pressure is always lower than the systolic pressure.

Blood pressure is measured in units of millimeters of mercury (mmHg). The readings are always given in pairs, with the upper (systolic) value first, followed by the lower (diastolic) value.

So someone who has a reading of 132/88 mmHg (often spoken “132 over 88”) has a

  • systolic blood pressure of 132 mmHg, and a
  • diastolic blood pressure of 88 mmHg.

What is normal blood pressure, and when is blood pressure considered to be high?

Blood pressure is always measured on a number of different days and when you are at rest. If several of these measurements are too high, you are said to have high blood pressure, even if only one of the two – either the systolic or the diastolic one – is high. The medical term for high blood pressure is hypertension. In adults, blood pressure is considered to be normal under a systolic value of 140 mmHg and under a diastolic value of 90 mmHg.

When taking your blood pressure for the first time, it makes sense to measure the blood pressure in both arms, because it's sometimes high on only one side. The values that are higher are always the ones used for assessing blood pressure. After that it is enough to measure the blood pressure only in the arm that produced the higher reading. A person is considered to have high blood pressure if the systolic value is over 140 mmHg, the diastolic value is over 90 mmHg, or if both are higher than these readings.

High blood pressure itself usually goes unnoticed. Only if it is extremely high can it sometimes result in symptoms like dizziness or trouble seeing. Over the long term, high blood pressure increases your risk of cardiovascular problems like heart attacks, strokes, and heart and kidney failure. So if you or your doctor think you have high blood pressure, it's important to have your blood pressure checked regularly. If the readings are repeatedly too high, there are several different ways of lowering your blood pressure and decreasing the risk of long-term health consequences.

Table: Normal and high blood pressure readings

Normal blood pressure

systolic under 140 mmHg and diastolic under 90 mmHg

High blood pressure

systolic over 140 mmHg and/or diastolic over 90 mmHg

How is blood pressure measured?

It's important to measure blood pressure more than once because it fluctuates over the course of the day. It can also change due to things like physical exertion, stress, pain, or extreme heat or cold. But this kind of increase in blood pressure is only temporary and it soon returns to normal.

So, if blood pressure is measured just once and found to be high, it doesn't necessarily mean that it's always too high. A blood pressure reading taken at the doctor’s office can also be misleading: Going to the doctor makes some people so nervous that their blood pressure goes up.

So to get reliable readings, blood pressure is measured on several different days and while you are resting. This means sitting down and relaxing on a chair, and waiting about three minutes before taking a measurement so that your circulatory system comes to rest. The upper arm that is being used for the measurement should rest on a table, at about the same height as the heart, while the reading is being done.

You can measure your blood pressure on your own using a digital blood pressure monitor for automated readings or an instrument called a sphygmomanometer for manual readings.

Digital blood pressure monitors

Digital blood pressure monitors are often used on the wrist, but they can also be placed on the finger or upper arm and are activated simply by pressing a button. They read the blood pressure automatically based on variations in the volume of blood in the arteries. When taking blood pressure measurements on the wrist, it's important to keep the hand level with the heart. Otherwise it can affect the readings.

Digital meters can sometimes be inaccurate and produce unreliable readings anyway – especially in people with certain heart rhythm problems or arteries that have hardened due to arteriosclerosis.

Measuring blood pressure with a sphygmomanometer

A sphygmomanometer has three parts:

  • a cuff that can be inflated with air,
  • a pressure meter (manometer) for measuring air pressure in the cuff, and
  • a stethoscope for listening to the sound the blood makes as it flows through the brachial artery (the major artery found in your upper arm).

The scale of the pressure meter ranges from 0 to 300 mmHg. The pressure meter has a rubber pump on it for inflating the cuff and a button for letting the air out.

To measure blood pressure, the cuff is placed around the bare and stretched out upper arm, and inflated until no blood can flow through the brachial artery. Then the air is slowly let out of the cuff.

As soon as the air pressure in the cuff falls below the systolic blood pressure in the brachial artery, blood will start to flow through the arm once again. This creates a pounding sound when the arteries close again and the walls of the vessels hit each other after a heart beat. The sound can be heard by placing the stethoscope close to the elbow. Right when you start to hear this pounding for the first time you can read your systolic blood pressure off the pressure meter.

The pounding sound stops when the air pressure in the cuff falls below the diastolic blood pressure in the brachial artery. Then the blood vessels remain open. Right when the pounding stops, you can read the diastolic blood pressure off the pressure meter.

Where can I learn how to take my blood pressure myself?

In Germany and other countries, people with high blood pressure can attend patient education courses that teach a number of things, including how to measure your blood pressure. As part of specialized disease management programs (DMPs) for people who have narrow coronary arteries (coronary artery disease, CAD), statutory health insurers offer additional healthcare services. These include patient education about high blood pressure. Some doctor’s practices don't offer these courses, though.

What is ambulatory blood pressure monitoring?

If a doctor recommends ambulatory blood pressure monitoring, you will need to wear a blood pressure cuff for 24 hours. It's connected to a small, portable measuring device that automatically measures your blood pressure at set times and records the readings.

Ambulatory blood pressure monitoring is used, for example, to see whether blood pressure levels vary greatly over the course of the day and night or whether they are constantly elevated. During those 24 hours, you can do all of the usual things you would otherwise do over the course of the day. If you are especially active at certain times, you can make a note in a diary. Then the doctor has a better idea of how to interpret the recorded values when evaluating them.